Shrinking integrated circuits (ICs) result in improved performance, increased capacity and/or reduced cost per transistor. Each device shrink requires more sophisticated techniques to form the features. Photolithography is commonly used to pattern features on a substrate. An exemplary feature is a line of a material which may be a metal, semiconductor or insulator. Linewidth is the width of the line and the spacing is the distance between adjacent lines. Pitch is defined as the distance between a same point on two neighboring lines. The pitch is equal to the sum of the linewidth and the spacing. Due to factors such as optics and light or radiation wavelength, however, photolithography techniques have a minimum pitch below which a particular photolithographic technique may not reliably form features. Thus, the minimum pitch of a photolithographic technique can limit feature size reduction.
Self-aligned double patterning (SADP) is one method for extending the capabilities of photolithographic techniques beyond their supposed minimum pitch. Such a method is illustrated in FIGS. 1A-1F. With reference to FIG. 1A, mandrels 102 are formed from sacrificial structural material above a dielectric layer 114 on a substrate 100 using standard photolithography and etching techniques. The mandrels 102 may have linewidths and/or spacings near the optical resolution of a photolithography system using a high-resolution photomask. As shown in FIG. 1B, a conformal layer 106 of hard mask material is subsequently deposited over mandrels 102. Hard mask spacers 108 are then formed on the sides of cores 102 by preferentially etching the hard mask material from the horizontal surfaces with an anisotropic spacer etch. The resulting structure is shown in FIG. 1C. Mandrels 102 may then be removed, leaving behind hard mask spacers 108 (FIG. 1D). At this point hard mask spacers 108 may be used as an etch mask for transferring the pattern to the layer to-be-patterned 114 to form patterned features 116, as shown in FIG. 1E. The hard mask spacers 108 may then be removed (FIG. 1F). Refinements to this process flow are needed to provide for greater consistency in the properties of hard mask spacers 108 and/or patterned features 116.